1. Field of the Invention
The present invention relates to an exhaust apparatus for an internal combustion engine. More particularly, to an exhaust apparatus for a multi-cylinder internal combustion engine.
2. Description of Background Art
It is known in regard to an internal combustion engine that a dynamic effect of such as inertia or pulsation of exhaust gas flowing in an exhaust pipe has an influence on the volumetric efficiency of the internal combustion engine. A multi-cylinder internal combustion engine has been proposed wherein the dynamic effect is applied. In particular, in the multi-cylinder internal combustion engine, exhaust pipes extending from corresponding cylinders are joined together at a position of a suitable length. Further, the exhaust timings of the cylinders are displaced from each other (actually the ignition timings of the cylinders are displaced from each other) so that a dynamic effect of pulsation or the like obtained from one of the exhaust pipes acts upon the other side cylinder through the exhaust pipe to achieve improvement of the volumetric effect of the other side cylinder.
In this instance, the volumetric efficiency differs depending upon the rotational speed of the internal combustion engine, and in a rotational speed range adapted to the set exhaust system, the aforementioned dynamic effect acts effectively to raise the volumetric efficiency. However, in some rotational speed range, the dynamic effect and the exhaust timing are sometimes displaced to conversely give rise to a drop of the volumetric efficiency. This is because, if the exhaust system is set in shape and dimension so that an effective dynamic effect may be obtained in a high rotational speed range, then when the rotational speed of the internal combustion engine reaches a medium rotational speed range, good matching between the exhaust timing and the dynamic effect is lost. This results in a drop of the volumetric effect of the internal combustion engine, and consequently, a phenomenon that the output power of the internal combustion engine drops in the medium rotational speed range. Therefore, a technique has conventionally been proposed to provide a changeover valve at the joining region of the exhaust pipes. The changeover valve is changed over in response to the rotational speed of the internal combustion engine to change the joining mode of the exhaust pipes so that a pulsation effect of exhaust gas in the rotational speed range in that the output power drops is canceled thereby to moderate the phenomenon that the output power drops.
As a measure for changing the joining mode of the exhaust system, a technology is disclosed, for example, in Japanese Utility Model Laid-Open No. Hei 2-28514. According to this technology, a four-cylinder internal combustion engine includes a bypass pipe that communicates each two of the exhaust pipes of a four-cylinder internal combustion engine with each other in a region wherein the four exhaust pipes are independent of each other. An on-off gate valve is provided in the inside of each bypass pipe and is driven to open and close. The bypass pipes are disposed at positions displaced forwardly and backwardly from each other to avoid interference therebetween and include the two on-off gate valves, and also two driving sources.
In this manner, in a conventional exhaust apparatus of the type described, an on-off gate valve is provided for each bypass pipe that communicates exhaust pipes with each other. Further, since also a driving source for driving an on-off gate valve is required for each of the on-off gate valves, there is a problem wherein an increase in the number of manufacturing steps is needed together with an increase in the cost and an increase of the number of parts.